Drag Flashcards
Drag definition
Drag: force which opposes the forward motion of an airplane
●opposes thrust
●equal to thrust in unaccelerated, straight-and-level flight
Two main forms of drag:
●Induced – Product of Lift
●Parasitic – Everything else
Induced drag
• Inversely proportional to the square of the speed. If speed is decreased by half, induced drag increases fourfold
• Increases total drag as airspeed decreases
• Can be directly controlled by the pilot
•Speed
•Flaps
•AOA
At low speeds a wing must fly at a higher AOA; result, more high pressure air from the lower surfaces comes around (or spills over) the wingtips, forming powerful vortices called — WING-TIP VORTICIES
At high speeds there are correspondingly less powerful wingtip vortices and less induced drag
Wingtip vortices
●a by-product of lift
●caused by the joining of the low and high pressure areas from above and below the wing
●spiraling vortex is generated
Infinite wings
•No Wing Tip Vortices
•No Down Wash
•No Induced Drag
Finite wings
•Wing Tip Vortices
•Downwash
•Induced Drag
•Must Operate at Higher AOA to Produce Same Lift as Infinite Wing
•Less Lift At the Same AOA as Infinite Wing
What determines induced drag
•Velocity
•Air Density
•Lift Required
•Characteristic of Wing
Rectangular wing
•Area Does Not Match Lift Distribution
•Root Works Hardest
•Higher Local AOA
•Tips Work Least
•Lowest Local AOA
Elliptical wing
•Area Matches Lift Distribution
•Airfoil Sections Work Equally
Droop tips
•Downward Deflection of Spanwise Flow
•Vortex Forms Farther Away
Endplates
•Block Airflow Around Tip
•More Parasite Drag
Winglets
Most effective at Low Speed and/or High Angle of Attack
Weight
•Induced Drag Increases with the Square of Weight
•Big Factor
Span efficiency factor
•Induced Drag Decreases with More Efficient Wings
•Elliptical = 1
•Others = .85 to .95
Wing span
•Induced Drag Decreases with the Square of Wing Span
•Big Factor
Air density
•Induced Drag Decreases with Greater Density
•Increasing Pressure
•Lower Altitude
•Colder Temperature